1. Field of the Invention
The present invention relates to low temperature separation of gaseous mixtures and is particularly concerned with enhancing recovery of ethylene and/or ethane from a mixture of hydrocarbons and low boiling gases containing the same.
2. Description of the Prior Art
Industrial plants for the recovery of C.sub.2 hydrocarbons, particularly ethylene from admixture with methane and/or propane and propylene (C.sub.3) and higher hydrocarbons are well-known. Such plants generally employ as part of the separation equipment a demethanizer column wherein the major portion of the methane content of the feed gas thereto is distilled as vapor overhead together with hydrogen, nitrogen, and any oxides of carbon that may be present. Practically all of the ethane and most of the ethylene are recovered as liquid in the bottoms fraction of the distillation column, together with C.sub.3 and higher hydrocarbons. A small portion of the ethylene, however, present in the feed to the demethanizer distills over with the vapor fraction. In the conventional overhead reflux condenser associated with the demethanizer column, part of the ethylene in the overhead is condensed and returned to the column as reflux; a minor but valuable portion of the ethylene in the overhead, which may be as much as 20% or more, is not condensed and passes over from the reflux condenser with the methane vapor fraction. By provision of additional partial condensation approximately 10% of the ethylene remaining in the vapor leaving the main reflux condenser can be recovered.
The use of a typical demethanizer column in a system for recovery of C.sub.2 hydrocarbons from a lean natural gas stream having a small C.sub.2 + content (in the order of about 3.5 mol %) is illustrated, for example, in U.S. Pat. No. 3,360,946. The tabulated material balance of the patent discloses that only about 68% of the C.sub.2 hydrocarbons are recovered. In a companion patent, U.S. Pat. No. 3,359,743, wherein the tail gas from the main reflux condenser of the demethanizer is further rectified, additional recovery of C.sub.2 hydrocarbons is obtained; 8.7% of the C.sub.2 hydrocarbons, however, are present in the final tail gas.
Demethanizer columns commonly employ low pressure ethane or ethylene as refrigerant to condense the overhead reflux. In some instances, Joule-Thompson (J-T) expansion refrigeration obtained from the overhead vapor product is utilized to provide auxiliary reflux, by partial condensation of the vapor leaving the main reflux condenser. In the case of a demethanizer column employed in an ethylene plant, such auxiliary partial condensation permits recovery of approximately 10% of the ethylene remaining in the vapor from the main condenser. Such processes are relatively inefficient in terms of the amount of ethylene recovered (10%) due to the extremely large amount of methane which must be condensed in order to recover the residual ethylene.
Various attempts to increase recovery of ethylene and/or ethane from demethanizer column overhead are disclosed, for example, in U.S. Pat. Nos. 3,262,278; 3,675,435 and 3,702,541.
Dephlegmators have been employed in the separation of binary gas mixtures, particularly where the components have widely separated boiling points, as exemplified in U.S. Pat. No. 2,582,068. Also in U.S. Pat. No. 4,002,042, directed to the recovery of the major portion of a C.sub.2 + hydrocarbon fraction from a feed gas mixture comprising methane and hydrogen, the feed gas is introduced into a dephlegmator where it is cooled to form a condensate stream containing the major portion of the C.sub.2 + hydrocarbons. The condensate stream, together with prior condensed material from the feed gas is passed to a demethanizer column. The uncondensed portion of the overhead vapor from the demethanizer column reflux condenser is utilized as one of the refrigerant streams in the dephlegmator.
Pending U.S. patent application Ser. No. 064,232, filed Aug. 6, 1979 discloses use of a dephlegmator for condensation and recovery of methane from ammonia plant purge gas or other industrial gas mixtures which contain hydrogen and one or more other low-boiling gases such as nitrogen and/or argon.